Duration: 3 months
Supervision: Artem Basyrov, Josquin Errard, R. Stompor
Host institute: APC, Université Paris Cité / CNRS
Scientific context
Measurements of the polarization of the Cosmic Microwave Background (CMB) provide a unique probe of the physics of the early Universe. In particular, current and future experiments such as the Simons Observatory and LiteBIRD aim at measuring the faint B-mode polarization signal, potentially generated by primordial gravitational waves.
Reaching the required sensitivity requires exquisite control of instrumental systematic effects. One important source of such effects is the instrumental beam, which describes how each detector observes the sky. While beams are often approximated as circularly symmetric, realistic beams can exhibit ellipticity and more complex asymmetric features. Combined with the scanning strategy of an experiment, these asymmetries can distort the reconstructed temperature and polarization maps and potentially generate spurious polarization signals.
Understanding these effects requires realistic simulations starting from the sky and propagating the signal through the instrumental response to produce time-ordered data (TOD), i.e. simulated detector measurements as a function of time.
Internship project
The goal of the internship will be to develop and validate simulations of CMB time-ordered data including asymmetric instrumental beams, building on simulation and map-making tools currently developed at APC.
The students will first become familiar with CMB polarization, scanning strategies and the generation of time-ordered data from simulated sky maps. Starting from a simple symmetric Gaussian beam model, the instrumental response will then be generalized to include controlled beam asymmetries, initially considering elliptical Gaussian beams.
The simulated observations will use scanning strategies representative of experiments such as the Simons Observatory and/or LiteBIRD. The resulting TODs will be projected back onto maps in order to study how beam asymmetries propagate into reconstructed temperature and polarization signals.
Depending on progress, the project may investigate effects such as temperature-to-polarization leakage, distortions of E- and B-mode polarization, and the dependence of these effects on the scanning strategy and beam parameters.
Possible work plan
The project will approximately consist of:
- familiarization with CMB polarization, instrumental beams and TOD simulations;
- implementation or integration of asymmetric beam models into the existing simulation framework;
- validation using simple test cases and comparison with the symmetric-beam limit;
- generation of simulated TODs for a realistic scanning strategy;
- reconstruction of maps and characterization of beam-induced residuals;
- if time permits, preliminary assessment of their impact on CMB polarization power spectra.
The internship will involve numerical simulations and scientific programming, primarily in Python. It will provide an introduction to the analysis challenges of current CMB experiments and to modern high-performance tools used for CMB data analysis.
Profile
The project is suitable for Master-level students with a background in physics, astrophysics or applied mathematics. Basic knowledge of Python and numerical methods is expected. Previous knowledge of cosmology or CMB physics is useful but not required.